This is a good question. "souna" means "that look(s)..." or "that sound(s)" as you mentioned and this term is always used after an adjective (or sometimes a verb) and in front of a noun, for example, "oishi-souna okashi" a cake that looks delicious "kanashi-souna kyoku" a music piece that sounds sad Here, be careful to take off the last "-i" of a Japanese adjective. For example, "oishii-souna" is incorrect. "soudesu" is a predicative form and usually put in the end of a sentence. "soudesu" has two usages: "look(s)" or "sound(s)"; and "they say...". Interestingly, these usage are distinguished by the conjugation of the precedent adjective, for example, "kono okashi wa oishi-soudesu" means "this cake looks delicious". Similarly to "souna", the last "-i" of the preceding adjective muste be taken off in this case. On the other hand, if you leave this "-i" as in "kono okashi wa oishii soudesu", then this phrase means "they say this cake tastes good."